Just tried to play Cure Space normal, joined the queue, and was added to a mission that had just finished. Then after leaving the map, I was stuck with the cooldown. Sadly, I didn't get any rewards for it either
The previous group finished Cure too quickly, and the instance sticks around. The remaining spot is then filled from the queue. It's been a problem for a while now.
The workaround so far is to watch the queue numbers. If it offers you a match while the queue is 0/5, for example, that means you're about to be added to an existing instance, and you should decline that spot.